open a new view is broken

[fcollonval]

Description

#12349 breaks the clone view command because the opener is no more defined in the plugin adding the commands

Reproduce

1. Open a notebook
2. Right click on the notebook tab
3. Select New view for notebook
4. Nothing happen an error is emitted in the console

Expected behavior

No error
It seems the solution would be to provide the opener as a plugin to be requested by the plugin adding the commands.

Context

• JupyterLab version: master

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